96 FLD120, 750k miles, 1 month old drives, 1 day old steers, ALL balanced

Driver told me the truck had a bad vibration. We had put new drives on about 1 month ago and I was concerned we may have had a bad one in the batch. He stopped at the same tire place and we had them balance (spin) all the drives. They noticed the steers were wearing bad - driver side cupping on inside and pass side outer tread wear. Took truck to very reputable alignement shop and got 3-axle alignment. Align guy said rears were out and causing the wear, but the front was fine. ... only charged me for aligning the rears. He did say that the driver steer was out-of-round and was prob causing the bounce.

I picked up the truck yesterday from the align shop and drove it about 15 miles to the tire shop to get the steers replaced. Truck (bobtail) would about bounce you out of the seat between 55-65mph and smooth out a tad over and below that. Tested pushing the clutch in and putting tranny in N while bouncing and no change - trottle on/off during had no change - bounce is roadspeed specific. Had new steers put on (I have Centramatics on steers) and drove it about 15 more miles and no change in the bounce. Had someone else drive it while I followed and it *looks* from what little I could tell going down the road, that the front rear is the one bouncing - back rear looks solid going down the road. Took it back today and had the Centramatics taken off and steers balanced and still no change.

Freightliner shop said just now that "on the driver side front rear is a bracket bolted to the frame, you measure from this bracket down to the spring hanger and should be 2 7/8" for proper ride height". I called and told my driver and he went out and checked but said 5" at first and then said the rear axle was about 2 7/8". I am not sure if he has found the right bracket to measure from.

Anyone have any suggestions on what could cause the bounce?

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That is not the correct procedure for ride height.

Ride height being out will cause a bounce, definitely. Go down the road, and dump the air for 5 seconds. If it gets worse or better your ride height is out.
Follow the correct procedure for ride height. Check all driveshaft angles as well.


It could also be a tire that wasn't mounted correctly on the wheel.

 

Hmmm.....after reading your reply I started to second guess what the guy told me and remembered that I had found and downloaded some Freightliner Maint .pdf files a while back. Found in the Heavy-Duty Trucks Maintenance Manual the following..........

On tandem (dual drive) rear axle configurations, measure the distance from the bottom of the forwardmost left axle stop to the top of the axle U-bolt pad. The correct distance for single and dual-drive rear axles is between 2-3/8" and 2-7/8".

...... and it has a picture of checking from the bumpstop to the top of the u-bolt pad. :?: I had my doubts that the measurement was something that simple. Maybe he was measuring at the right spot. I'm gonna go down there tomorrow and see for myself.

If so........... any more ideas?

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Well, worked on the truck some more today and checked and set ride height as described in the manual. It was at about 2 3/8" and I set it to 2 7/8" or 3". Didn't make any difference at all in the bounce. Checked the drivelines & u-joints and they all seemed fine. I was having the truck serviced at this time, so both me and the mechanic checked them.

I had followed the truck bobtail and suspected the bounce was coming from the driver front drive tires. Discussing it today and the driver mentioned that he had a friend follow him and his friend mentioned the same thing...... so we decided to swap the outside tire/wheel on that side with the outside wheel on the pass rear drive. Doing this seemed to make a noticable difference in the bounce felt in the seat and also it seemed you could tell that the bounce was coming from the back pass corner now. Following the driver down the road bobtail after that and the rear axle now was bouncing where before it was smooth running. Seems I have a bad tire - out of round - or it is bad out of balance. Going to take it to the tire shop when the driver gets back from the load he has on and have them check balance and if it is in, replace the tire.

Just for info, tires are Yok RY617 steer and Yok TY517 drives.

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I'm going to say one tire was not seated correctly.

And it sounds like you need shocks.

 

I would check the shocks. Worn shocks can allow the tires to bounce. You may also want to check the king pins and tie rods. If they have any play in them it could cause excess tire wear.

 

Well, had the suspect tire checked out today at the tire shop - they spun it on the balancer and could not see that it was out of round or balance, and my driver was standing right there watching and confirmed that. (got a new driver and I trust his reports) Had him single that side out and bobtail the truck down the road without the suspect tire and he said it STILL has the bounce. I am at a loss.

The front shocks, kingpins, bearings, tie-rods, drag & pitman were all replaced about 1yr ago. Align shop said front was fine, and that rear axles were causing the wear we seen up front. Rear is in spec now.

The rear shocks *are* the stock ones, so at 750k I am sure they are beyond shot ........... maybe that's the culprit. Just hate to throw money at a problem without finding out the cause. But, there is no other way to confirm if the shocks are it than replacing them.......... :?
But, even if the shocks were removed all together, balanced tires shouldn't bounce like that, right? Wouldn't the shocks just mask an issue? Am I on the wrong line of thought there?


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i was in a diffirent truck for a few days untill mine got back from the shop... same exact truck but omg did this thing ride like complete shit. bouncing all over the place. almost snapped my neck hitting the roof. i wrote the truck up for a bad suspension they looked at it and said everything was in spec... im sorry but theres no way the thing is the way it should be. i got back in my truck and it was night and day... thank god thats not my truck. fl columbias

 

Does it feel like it's vibrating from under your seat or from the steering wheel? If it feels like it's coming from under the seat, you may want to check for flatspotted drives.

 

Don't waste your money on shocks. They do not stop or cause a vibration. Shocks are meant to dampen the compression and return of the suspension.

Logic would tell you that if your driving down a smooth road and you have a vibration or bounce, shocks are not the cause.

I guarantee you WILL NOT solve the problem with shocks.